Understanding Radiation: Fallout

Worldwide Fallout

After an air burst the fission products, un-fissioned nuclear material, and weapon residues which have been vaporized by the heat of the fireball will condense into a fine suspension of very small particles 0.01 to 20 micrometers in diameter. These particles may be quickly drawn up into the stratosphere, particularly so if the explosive yield exceeds 10 Kt.

They will then be dispersed by atmospheric winds and will gradually settle to the earth’s surface after weeks, months, and even years as worldwide fallout. The radiobiological hazard of worldwide fallout is essentially a long-term one due to the potential accumulation of long-lived radioisotopes, such as strontium-90 and cesium-137, in the body as a result of ingestion of foods which had incorporated these radioactive materials. This hazard is much less serious than those which are associated with local fallout and, therefore, is not discussed at length in this publication. Local fallout is of much greater immediate operational concern.

Local Fallout

In a land or water surface burst, large amounts of earth or water will be vaporized by the heat of the fireball and drawn up into the radioactive cloud. This material will become radioactive when it condenses with fission products and other radio contaminants or has become neutron-activated. There will be large amounts of particles of less than 0.1 micrometer to several millimeters in diameter generated in a surface burst in addition to the very fine particles which contribute to worldwide fallout.

The larger particles will not rise into the stratosphere and consequently will settle to earth within about 24 hours as local fallout. Severe local fallout contamination can extend far beyond the blast and thermal effects, particularly in the case of high yield surface detonations. Whenever individuals remain in a radiologically contaminated area, such contamination will lead to an immediate external radiation exposure as well as a possible later internal hazard due to inhalation and ingestion of radiocontaminants.

In severe cases of fallout contamination, lethal doses of external radiation may be incurred if protective or evasive measures are not undertaken. In cases of water surface (and shallow underwater) bursts, the particles tend to be rather lighter and smaller and so produce less local fallout but will extend over a greater area.

The particles contain mostly sea salts with some water; these can have a cloud seeding affect causing local rainout and areas of high local fallout. For subsurface bursts, there is an additional phenomenon present called “base surge.” The base surge is a cloud that rolls outward from the bottom of the column produced by a subsurface explosion.

For underwater bursts the visible surge is, in effect, a cloud of liquid (water) droplets with the property of flowing almost as if it were a homogeneous fluid. After the water evaporates, an invisible base surge of small radioactive particles may persist.

For subsurface land bursts, the surge is made up of small solid particles, but it still behaves like a fluid. A soil earth medium favors base surge formation in an underground burst.

Meteorological Effects

Meteorological conditions will greatly influence fallout, particularly local fallout. Atmospheric winds are able to distribute fallout over large areas. For example, as a result of a surface burst of a 15 Mt thermonuclear device at Bikini Atoll on March 1, 1954, a roughly cigar-shaped area of the Pacific extending over 500 km downwind and varying in width to a maximum of 100 km was severely contaminated.

Snow and rain, especially if they come from considerable heights, will accelerate local fallout. Under special meteorological conditions, such as a local rain shower that originates above the radioactive cloud, limited areas of heavy contamination may be formed.

U.S. Unprepared for Dirty Bomb
The U.S. has a shortage of laboratories to test the thousands of people who might be exposed to radiation if a “dirty bomb” detonated in a major city, according to a congressional report released Thursday.

Radioactive Fallout Will be the Killer
Like the more than 160 million Americans who live within the danger zones, your greatest concern following a nuclear attack comes from radioactive fallout. That's the main reason you will need a well-constructed, underground bomb shelter.

Bomb Shelter Writing Supplies
Are writing supplies available, including pens or pencils and printed forms or paper, for keeping records of radiation exposure?

Nuclear & Radiation Articles

Types of Nuclear Explosions
The immediate phenomena associated with a nuclear explosion, as well as the effects of shock and blast and of thermal and nuclear radiations, vary with the location of the point of burst in relation to the surface of the earth. For descriptive purposes five types of burst are distinguished, although many variations and intermediate situations can arise in practice.

Sources of Radiation
Blast and thermal effects occur to some extent in all types of explosions, whether conventional or nuclear. The release of ionizing radiation, however, is a phenomenon unique to nuclear explosions and is an additional casualty producing mechanism superimposed on blast and thermal effects.

Time Scale of a Fission Explosion
An interesting insight into the rate at which the energy is released in a fission explosion can be obtained by treating the fission chain as a series of "generations." Suppose that a certain number of neutrons are present initially and that these are captured by fissionable nuclei; then, in the fission process other neutrons are released.

Thermonuclear Fusion Reactions
From experiments made in laboratories with charged-particle accelerators, it was concluded that the fusion of isotopes of hydrogen was possible.

Thermal Radiation
The observed phenomena associated with a nuclear explosion and the effects on people and materials are largely determined by the thermal radiation and its interaction with the surroundings. It is desirable, therefore, to consider the nature of these radiations somewhat further.

Fission Products
Many different initial fission product nuclei, i.e., fission fragments, are formed when uranium or plutonium nuclei capture neutrons and suffer fission. There are 40 or so different ways in which the nuclei can split up when fission occurs; hence about 80 different fragments are produced.

Fission Energy
The significant point about the fission of a uranium (or plutonium) nucleus by means of a neutron, in addition to the release of a large quantity of energy, is that the process is accompanied by the instantaneous emission of two or more neutrons.

Critical Mass for a Fission Chain
Although two to three neutrons are produced in the fission reaction for every nucleus that undergoes fission, not all of these neutrons are available for causing further fissions. Some of the fission neutrons are lost by escape, whereas others are lost in various nonfission reactions.

Attainment of Critical Mass in a Nuclear Explosion
In order to produce an explosion, the material must then be made “supercritical,” i.e., larger than the critical mass, in a time so short as to preclude a sub-explosive change in the configuration, such as by melting.

Residual Radiation
The residual radiation hazard from a nuclear explosion is in the form of radioactive fallout and neutron-induced activity.

Radiation and Fallout
Radioactive fallout will fall in a manner similar to that following a volcanic eruption. It will be flaky in appearance and its size may reduce to dust particles or smaller. Expect it to be thicker near the detonation site and thinner as it travels down wind.

Initial Radiation
About 5% of the energy released in a nuclear air burst is transmitted in the form of initial neutron and gamma radiation. The neutrons result almost exclusively from the energy producing fission and fusion reactions, while the initial gamma radiation includes that arising from these reactions as well as that resulting from the decay of short-lived fission products.

General Principles of Nuclear Explosions
An explosion, in general, results from the very rapid release of a large amount of energy within a limited space. This is true for a conventional “high explosive,” such as TNT, as well as for a nuclear (or atomic) explosion, although the energy is produced in quite different ways.

Worldwide and Local Fallout
The radiobiological hazard of worldwide fallout is essentially a long-term one due to the potential accumulation of long-lived radioisotopes, such as strontium-90 and cesium-137, in the body as a result of ingestion of foods which had incorporated these radioactive materials.

Energy Yield of Nuclear Explosions
The “yield” of a nuclear weapon is a measure of the amount of explosive energy it can produce. It is the usual practice to state the yield in terms of the quantity of TNT that would generate the same amount of energy when it explodes.

Distribution of Energy in Nuclear Explosions
The basic reason for this difference is that, weight for weight, the energy produced by a nuclear explosive is millions of times as great as that produced by a chemical explosive.

Atomic Structure and Isotopes
A less familiar element, which has attained prominence in recent years because of its use as a source of nuclear energy, is uranium, normally a solid metal.

Informational Articles

Radioactive Fallout Will be the Killer
Like the more than 160 million Americans who live within the danger zones, your greatest concern following a nuclear attack comes from radioactive fallout. That's the main reason you will need a well-constructed, underground bomb shelter.

Bomb Shelter Writing Supplies
Are writing supplies available, including pens or pencils and printed forms or paper, for keeping records of radiation exposure?

Group Dosimetry: Keeping Track of Radiation Exposure
The radiation hazard will be worst throughout the first 24 hours after each fallout cloud arrives. It is important to start keeping track of everyone’s radiation exposure right away, as soon as fallout begins to arrive.

Time-Averaging Method
Used to compare the radiation levels between two or more locations in a bomb shelter when the radiation levels are climbing rapidly and when you have only one survey meter.

Space in the Bomb Shelter
Is there going to be enough room for all of the people at this bomb shelter in the locations of best protection?

Organization of the Bomb Shelter Population
Organization of the bomb shelter population into bomb shelter units, each with its own Unit Leader, is necessary not only for good management but also for keeping a radiation exposure record for each person in the bomb shelter.

Materials for Shielding the Bomb Shelter
You may have improved the radiation safety of the bomb shelter to the best of your judgment and capability, as discussed earlier. But after fallout arrives, you may find with the use of your survey meter that gamma radiation is shining through at some unexpected location.

Light Sources in the Bomb Shelter
Electricity may fail in many locations due to a wide-scale nuclear attack. Most of the bomb shelters with the highest FPF’s will also have the least daylight reaching them. If the power goes out, these bomb shelters may be pitch black.

Getting and Checking the Bomb Shelter Instruments
If you are selected to be an RM after you arrive at the bomb shelter, you may have to find out where the radiation instruments are, and you may have to make a special trip to get them. Instructions on how to use the instruments may be given at the place where they are issued.

Gamma Shielding by using People in the Bomb Shelter
The shielding effect of human bodies can be used to provide extra protection. This protection would be of particular benefit to those people with the greatest sensitivity to radiation, namely, children and pregnant women.

Forecasting Radiation Exposure
When the survey meter readings level off and then continue to decrease, the arrival of fallout from that particular cloud at your location has almost ended. If no more fallout clouds arrive, the radiation levels will continue to decrease rapidly.

Finding and Covering up Leaks in Bomb Shelter Gamma Shielding
After the safest locations have been found in the bomb shelter and the people have moved there (if they weren’t there already), use the survey meter to make detailed measurements of the radiation levels in and around the area where the people are located.

Dosimeter Locations: Where to Place Dosimeters
In some bomb shelters where the FPF is high and about the same everywhere, as in deep underground bomb shelters, caves, and mines, only a few dosimeters need to be mounted or hung where people will be located, to get an idea of what total exposures they are getting, if any.

Informational Articles

Checking Out the Bomb Shelter
Some bomb shelters may have many rooms, some of them on different levels, and others may have just one large room. The problems of providing the best radiation safety will be a little different in each bomb shelter.

Best Bomb Shelter Protection
Which locations within the bomb shelter appear to offer the best protection against fallout? Sketch a bomb shelter floor plan and mark these locations.

Bomb Shelter Openings and Ventilation
Are there openings to be baffled or covered to reduce the amount of radiation coming through them? Will these changes allow enough air to flow through to keep people from getting too hot when they are crowded?

Bomb Shelter Entranceway Problems
One problem that could develop is that the bomb shelter entrance could be blocked by people who have stopped just inside the entrance.

Minimizing Exposure to Radiation
It's people like you and me (hopefully) that will survive the initial blast. Our greatest concern is radioactive fallout. Fallout will kill as many, if not much more than the blast itself. And how long you have before fallout arrives depends on these three things.

Bomb Shelter Location
The location you choose for your bomb shelter should be one which gives you the greatest protection possible. Just placing an underground bomb shelter in your back yard is not enough.

Bomb Shelter Design
What should your underground bomb shelter look like? What materials should it consist of? How should it be designed? These are all important considerations when planning the construction of an underground bomb shelter.

Blast and Fallout Concerns
The blast wind produced by a nuclear bomb will reach 2,000 mph within the first half mile from ground zero, drop to about 1,000 mph at 2 miles, and will still be at hurricane force (200 mph) several miles out.

Before Fallout Arrives
It may not be possible to do all these tasks before fallout arrives at the bomb shelter or fallout shelter, and in that case, those tasks that can be done inside the bomb shelter can be done later while fallout is arriving.

Understanding Radiation
What is radiation, you ask? Radiation in physics is the process of emitting energy in the form of waves or particles. Various types of radiation may be distinguished, depending on the properties of the emitted energy/matter, the type of the emission source, properties and purposes of the emission, etc.